The present invention relates to the field of anomaly detection. More particularly, the present invention relates to the field of buoyant and submersible anomaly detecting transducers which produce an electrical signal in response to the detection of an anomaly. Even more particularly, the present invention relates to a method and apparatus for buoyant and submersible anomaly detection which are responsive to an anomaly in the earth's magnetic field corresponding to a modern nuclear or diesel submarine.
Anti-submarine warfare has historically used two methods for detecting the presence of submarines. The first method incorporates a plurality of buoys or "sonobuoys" which are adapted to descry the sound or "acoustic signature" of a submarine. The second method utilizes an airborne magnetic field sensor, which is towed behind an aircraft, for detecting disturbances in the earth's magnetic field. These magnetic disturbances may be caused by large, metallic, underwater objects such as a submarines. Both prior methods are outlined below with each having respective advantages and disadvantages.
The first method or "sonobuoy" method incorporates a plurality of buoys which are adapted to be positioned in a two dimensional, geographical relation, at or near the surface of the ocean. Each sonobuoy continuously monitors sonic vibrations received from within the ocean and continuously transmits information corresponding to the sonic vibrations as radio signals. Sophisticated sound detection equipment or "hydrophones" detect the acoustical vibrations. Once the radio signals are received onboard a nearby aircraft, they are analyzed by a technician. The sonic vibrations as received and analyzed may potentially correspond to the acoustic signature of a modern nuclear or diesel submarine.
The acoustic signature of a submarine is produced by the engine, propeller, and internal mechanics of the submarine as it passes through the water. Each sonobuoy, once deployed, transmits all of the acoustical information received. This acoustical information may include the acoustical signature of a submarine. However, the sonobuoy method suffers from a disadvantage in that the acoustical information must be continually monitored by technical operators onboard a nearby aircraft. This necessitates that each sonobuoy be assigned to its own respective radio frequency for transmitting its signal to the aircraft. Further, the technical operators onboard the aircraft must undertake extensive training to recognize the acoustical signature of a submarine apart from the surrounding background noise. Unfortunately, the sonobuoy system suffers from the added disadvantage that the number of sonobuoys which may be deployed depends upon the number of radio frequency channels available for transmission. For example, the equipment onboard the aircraft may only support a limited number of frequency channels, such as 50 to 100, thus limiting the number of channels and therefore the number of buoys available for use. Thus, the sonobuoy method affords coverage of a relatively predetermined geographical area of the ocean but is limited by the number of buoys available.
A number of prior systems have been developed which utilize the sonobuoy detection method for detecting the presence of an underwater vehicle such as a submarine. Sikora, U.S. Pat. No. 3,720,909, incorporated herein by reference, discloses a directional hydrophone buoy system which produces an electrical signal in response to the sound pressure emitted from an underwater sound source, such as the propeller of a submarine. In the hydrophone buoy of Sikora, the buoy is given appropriate ballast such that the buoy is submersed below the surface of the water. A mass including a coil winding is suspended by springs within a local magnetic field, which is produced by permanent magnets. Sonic vibrations from the surrounding ocean produce a corresponding movement of the coil within the magnetic field and therefore a voltage output corresponding to the rate at which the coil moves, i.e. the amount of sound detected.
Richard, U.S. Pat. No. 3,226,670, incorporated herein by reference, discloses an apparatus for determining characteristics of the ocean bottom. According to Richard, a small radio telemetering buoy is dropped into the ocean from an aircraft. When the buoy falls into the water, a bag disposed on a distal end thereof is inflated. A radio antenna is suspended within the inflatable bag. An echo sounding system is employed by first electrically detonating an explosive charge by way of a delayed sea water switch. A hydrophone disposed within the telemetering buoy then receives an impulse from the detonated charge and relays the information to a nearby aircraft via radio signals.
It thus appears that the sonobuoy method offers the advantage of placement over a relatively predetermined ocean area for submarine detection. However, the sonobuoy method suffers from the disadvantage of size limitation i.e., the acoustic detection equipment takes up a large physical area. The sonobuoy method also suffers from the requirement of a separate radio frequency channel for each sonobuoy, and the requirement that highly trained operators must continually monitor all received signals from the sonobuoys. Additionally, the limited number of radio channels available onboard the aircraft effectively limits the number of sonobuoys deployed and thus the geographical area of coverage.
The second method for detecting submarines utilizes an airborne magnetic field sensor which is suspended behind an aircraft such as an airplane or helicopter. The airborne magnetic field sensor detects an anomaly in the earth's magnetic field and then communicates this information to the aircraft for analysis. Airborne magnetic field sensors have also been employed for detecting geological noise effects produced by the varied shape of the sea floor. The airborne sensor method generally requires a large power source for detecting the presence of a submarine far below the ocean surface.
The airborne sensor method provides the advantages of maneuverability and that the course of search may be readily changed. Further, this method is not limited by the number of buoys that may be housed by an aircraft. However, this method suffers from the disadvantage of limited swath width and thus cannot readily and simultaneously cover a wide two dimensional area.
Murphy, U.S. Pat. No. 2,632,884, incorporated herein by reference, discloses an orienting mechanism for magnetic detector devices or magnetic field sensors. As disclosed, a body or bird is towed behind an aircraft by way of a cable or the like. A magnetic field sensor having three mutually perpendicular axes is enclosed within the bird. One of the sensors or coils is selected as a detector coil and is adapted to be maintained in alignment with the lines of force of the earth's magnetic field, for example by servo motors. The other axes are placed mutually perpendicular thereto.
Each of the airborne magnetic field sensors must be supported by a dedicated aircraft. Thus the magnetic field sensor method offers the advantage of portability, however it provides a relatively narrow area of coverage when compared with the sonobuoy method.
Heretofore, the range of sound detection has been far superior to that of magnetic anomaly detection, given the same power requirements. However, with the advent of quieter and quieter submarines, the effective range of sonobuoy detection has become considerably weakened. These technological advances in the art of quieting submarines have not been economically addressed by the advances in the art of acoustical search sensing. Thus, modern submarine detection has become an increasingly difficult exercise.